322 COMPARATIVE ZOOLOGY OF CHORDATES 



In the more highly modified of these paddle-like limbs (as in 

 the whales, for example), it is common to find that the number 

 of phalanges is increased (a condition known as hyperphalangy). 

 In the broad paddles of Ichthyosaurs, the number of rows 

 of phalanges exceeds five, producing the condition called 

 hyperdactyly. 



The 5th metatarsal bone is an object of interest. Normally 

 this bone is straight, as in the amphibia, the most primitive 

 reptiles (Cotylosauria), the Theromorph and allied reptiles 

 and the mammals. In other groups of reptiles, however, it 

 is peculiar in being hook-shaped, and the possessors of this 

 modified type of 5th metatarsal are : Sphenodon, lizards, 

 tortoises, crocodiles, Dinosaurs, and Pterosaurs. It is worthy 

 of note that these groups all have characters in common in the 

 structure of the heart or of the skull, and are regarded as 

 belonging to the great Sauropsidan branch of the reptiles which 

 culminates in the birds. It is probable that the hook-shaped 

 metatarsal is characteristic of this group, and distinguishes it 

 from the other main stem of reptiles (Theropsida) which evolved 

 in the direction of mammals. The evidence from the 5th 

 metatarsal fits in with that obtained from other sources. 



Mention must be made of those animals which have lost 

 their limbs. They have totally disappeared in some of the 

 eels. Among the amphibia, the pelvic limbs and girdle have 

 been lost in the Sirenidae, and the worm-like Gymnophiona 

 have lost all the limbs and girdles. Coming to the reptiles, the 

 snakes have lost the girdles and the pectoral limbs altogether, 

 while only very small vestiges of the pelvic limbs remain. 

 Several families of lizards have independently assumed the 

 snake-like form by loss of the limbs, such as the slow- worm 

 (Anguis), some of the Scincidae and the Amphisbaenidae. 

 These forms furnish an interesting example of convergent 

 evolution. Among mammals the pelvic girdle and limbs 

 vanish almost completely in the whales (Cetacea) and Sirenia. 



Literature 



Goodrich, E. S. Vertebrata Craniata, Cyclostomes and Fishes. Black, 

 London, 1909. 



